This invention relates to electronic devices, for example an active-matrix liquid-crystal display or image sensor or a data store, comprising thin-film circuit elements which are connected together on a substrate to form a thin-film circuit, for example a drive circuit for a group of device cells. The present invention relates to improving the performance of the thin-film circuit.
Large-area active matrix devices were manufactured for many years with the device cells formed as thin-film circuit elements on an insulating substrate and with drive circuits formed as monolithic silicon integrated circuits. The monolithic drive circuits may be mounted on the periphery of the substrate or beside the substrate and required a large number of electrical connections to be made to thin-film circuit terminals on the substrate.
In more recent years drive circuits have been formed as thin-film circuit elements integrated on the same insulating substrate as the thin-film device cells. This thin-film integration of the drive circuit offers a number of advantages which include: a more compact device; a reduced complexity of electrical interconnection; and a possibility of reducing manufacturing cost. Thus, if the drive circuit is fabricated using thin-film circuit technology compatible with that used to make the active devices, then the drive circuit could in principle be produced on the device substrate at little or no additional cost (assuming that the integration does not significantly affect the yield of working devices).
Published European patent application EP-A-0 342 925 describes some examples of the integration of thin-film drive circuits on an active matrix device substrate, for example for a flat-panel liquid-crystal display. Thus, EP-A-0 342 925 describes both column and row drive circuits, each having an input terminal on the substrate for supplying an input signal to the drive circuit, and having a succession of parallel output stages which are coupled to the device cells and which serve sequentially to supply signals at respective parallel nodes of the output stages for driving the device cells. Both the column and row drive circuits of EP-A-0 342 925 comprise a shift register for sequentially addressing the output stages. The row drive circuit may include a buffer circuit at its outputs to the matrix. The column drive circuit includes sample-and-hold circuits at its outputs to the matrix.
However, thin-film drive circuits have inferior performance as compared with monolithic integrated circuits, because the thin-film circuit elements are formed with polycrystalline (or even amorphous) silicon as compared with the monocrystalline silicon with which conventional monolithic circuits are formed. Thus, a silicon thin-film transistor (TFT) generally has a smaller ON current, a larger OFF current and a slower switching speed as compared with a monolithic silicon field-effect transistor. TFTs often exhibit high threshold voltage and low mobility, and this makes it difficult to design high performance circuits with TFTs. These inferior characteristics are understood to result from a high density of trapping states for charge-carriers in the TFT thin film. EP-A-0 342 925 seeks to reduce the effect on performance by forming the drive circuit with complementary TFTs (i.e. both n channel and p channel TFTs) in some embodiments.